Twitter hasn't seen unique visitor growth since July, but this happened last year. Nonetheless, can flatness justify $4 billion?

For Twitter, it isn’t good enough to be big – it also has to continue its rapid growth. With 175 million users and 25 billion tweets, we know Twitter is substantial. It has the power to move markets and humanitarian efforts, and it’s almost universally acknowledged that this microblogging platform is changing the nature of journalism.

But, Twitter has yet to make money, especially to justify the rumored $4 billion valuation. If Twitter wants to buy time, it needs to keep posting big growth rates, as that keeps alive the prospect of future monetization. Essentially, it means that potential is still in play. When potential has run its course, Twitter needs to generate actual money for its investors.

According to TechCrunch, it looks like Twitter’s growth is stalling. The company picked up around 24 million U.S. unique visitors last month, down from 25.1 million in October. While monthly unique are up 24 percent year over year, TechCrunch says, there hasn’t been any growth since July. This may be a normal business pattern, however, as there was concern about second-half growth for the company at this time last year. Prospects do look better on the global front: worldwide monthly uniques reached 104.6 million for Twitter in October, up 79.4 percent year over year.

Do note that TechCrunch is using Twitter.com traffic as a proxy for overall company health. Many users access the social media platform through devices or Twitter clients, such as HootSuite.

So, what does all this traffic mean? At 24 million unique, Twitter is roughly equivalent to Yelp in size, and it’s smaller than LinkedIn, which garnered 28.5 million uniques. Interestingly, Twitter is also bested by Huffington Post, with 26 million U.S. unique. It lags Facebook, which pulled in 152 million uniques, TechCrunch reports.

And this is where things get interesting, particularly when you look at Huffington Post.

Content companies, it seems, don’t score the same valuations as social media platforms. Aol picked up TechCrunch for around $40 million earlier this year, it’s been reported. The Gawker network’s value has been estimated (by Nick Denton) to be around $30 million, and his company is a little behind Twitter in monthly U.S. uniques.

Yet, Twitter is said to have attained a $4 billion valuation.

Granted, Twitter brings more to the table than traffic, but the high price tag still seems to be a stretch, especially given the company’s search for a business model this year. The chasm between Twitter and content sites with commensurate U.S. traffic is too great to ignore.